Owlpen Manor – Render Repairs

The following is a description of work carried out by JBLimeplastering at Owlpen Manor in Gloucestershire

The masonry revealed by the removal of the cement render was very friable with missing elements. The building mortar used in the construction was predominantly mud/loam mortar and so had suffered badly under the heavy, non breathing render. Despite the alarming appearance, quick progress was made to fill the voids and build up the surface (dubbing out) with stone inserts replacing missing masonry elements and broken terracotta tiles used in conjunction with well haired lime mortar to build up hollows to create a more even surface suitable for applying a trowelled lime render. Once the surface was repaired an initial coat of lime mortar was applied by trowel to an average thickness of 15mm. This coat was diagonally scored to provide a good key for the second coat. Care was taken to build up the surface in order to remove some of the undulations. Extra compaction was applied by beating with a bundle of withies (thin flexible twigs) in order to provide extra key but primarily to compact and counteract shrinkage thereby achieving greater strength. This first coat was left to cure. The second coat was again trowelled on and was similar in nature to the first coat. However the surface was scored with a stiff brush rather than scored to provide a key for the roughcast coat – some further beating with the withies took place. The final render coat was thrown on (roughcasting or harling) to provide a weather resistant rough crust. This coat was applied in a semi fluid state and so is no more than 8mm thick. Care was taken to achieve a uniform pebble textured surface. Once this final textured surface was dry a coat of unpigmented limewash was applied by brush. Two more coats of coloured limewash were applied with brushes before the winter break with the intention of returning after the winter to apply a further coat. During a severe winter immediately following the project a few small patches of surface damage required repairing before the final limewash coat was applied. Mixtures Repair mortars and trowelled coats: One part Natural Hydraulic lime NHL2 Two and a half parts limestone dust from Wickwar quarry (5mm to dust) Horse hair: A generous handful to two cubic feet of mortar Roughcast mix: One part NHL2 Two and a half parts South Cerney grit sand. Limewash was made from lime putty: Pigments: The main colour used was Yellow ochre (coded Y42) Modern pigments are too vibrant and rich to achieve a correct vernacular result and so the tones have to be tempered by the addition of Umber: Raw Umber, as used here, imparts a cool tone trending towards green whilst Burnt Umber imparts a warmer tone trending towards red. Notes on the Mortar mixtures. A decision had to be made regarding the type of lime to be employed. A large consideration was the time of year and exposure of the gable. These factors ruled out the use of lime putty as the binder due to its slow setting rate and its vulnerability in the first few months after application and so I chose Hydraulic lime as the binder. I was very conscious of the condition of the masonry and felt that I needed to try and achieve a render that was tough and weather resistant without being too hard and unbreathing. With this in mind I selected a local limestone dust as the aggregate – the nature of which created an easy flowing mortar. Hair was added in generous quantities to add tensile strength to the render which would be kept as thin as possible, I felt it was important to keep the weight of the render to a minimum – a thin render will carbonate quicker, shrink less and stay attached to the wall longer. JB

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About byrnesurfaces

conserver and repairer of historic surfaces
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